In some hybrid four-wheel-drive systems, driving force for one of the front or rear wheels is provided from an engine and driving force for the other of the front and the rear wheels is provided from an electric motor.
For example, a vehicle may have the engine and a generator in the front and drive the front wheels with driving force of the engine. Furthermore, the vehicle may have a motor in the rear and drive a rear wheels with driving force of the motor. Because the rear wheels are powered only by a motor, the motor must be large.
For example, other four-wheel-drive vehicles may have the engine in the front and a transaxle system powertrain with the transmission placed in the rear, near the rear axle. In this configuration, engine power passes through the driveshaft into the transmission, which is integrated with the rear final reducer, passing through the rear final reducer from the gearshift output to drive the rear-wheel driveshaft. The gearshift output again passes through the driveshaft into the front final reducer to drive the front-wheel driveshaft. Generally, the transmission in the rear of the vehicle increases weight on the rear axle, raising the slip threshold of the rear wheels, which are the drive wheels, and increasing the driving force on the road surface.
However, if the four-wheel-drive vehicle is equipped with a regular transfer case for rear-wheel drive, driving force distribution controllability is limited due to distribution of torque from the engine, the torque first going to the rear axle, then to the front-wheel driveshaft. Therefore it is difficult to distribute a greater driving force to the front wheel than to the rear wheel.
Therefore, in consideration of the above-mentioned problems, the purpose of this invention is to provide a hybrid four-wheel-drive system with increased controllability of the driving force distribution to the front and rear wheels.